The availability of telecommunication networks--i.e., the possibility for a user to transmit and receive data through a telecommunication network at any instant despite failure at one element of the network--is of great interest in the telecommunication field. The telecommunication users are particularly sensitive to the response time involved in their communication through the network and generally speaking, a response time of about one minute is considered as being a maximum acceptable delay for high priority interactive services. Such a requirement implies that the telecommunication network be capable of managing the different failures and outages which are likely to occur in the telecommunication network and which are likely to interrupt the telecommunication.
FIG. 1 shows an example of a System Network Architecture (SNA) network allowing the communication between host computers or servers 100 and 101 and remote Data Terminating Equipment (DTE) 110 via a X25 telecommunication network 109. Host computer 100, which can be, for example, an IBM 3090 computer, is connected to a local front-end communication controller such as an IBM 3745 via a telecommunication channel 102. Similarly, host computer 101 is attached to a second local front-end communication controller 106 via a second telecommunication channel 104 and can also, for instance, be attached to communication controller 105 via a third channel 103. Host computers 100 and 101 are loaded with their respective application programs such as credit card or airlines telecommunication applications. Communication controllers 105 and 106, being in the same location in the considered example, are loaded with Network Control Programs (NCP) well known in the telecommunication field. Front-end controllers 105 and 106 are connected via a SNA network 112 to a set of two remote telecommunication controllers 107 and 108. Telecommunication controllers 107 and 108 are loaded with Network Control Program (NCP) and also a telecommunication software--hereafter referred to NCP Packet Switching Interface or NPSI--for allowing the access to a X25 type network 109, thereby allowing the DTEs 110 to communicate with host computers 100 and 101. It should be noticed that the number of host computers which can be attached to the telecommunication network is not limited to two. Assuming that failure of an element of the telecommunication network occurs, such as a host, a channel, a link or any intermediate node communication controller, except however those providing access to the X25 network -the end-user will be compelled to wait until the considered element is repaired; this might take a long time before the user is allowed to communicate again through the network.
Moreover, in some applications such as those applications cited above, it is quite frequent that the hosts 100 and 101 share an unique data base which can be updated by either of them. Airlines reservations applications are frequently running in different host computers which can access the same data base by means of an appropriate software such as the IBM Transaction Processing Facility. In such cases, it can appear that the Communication and Transmission Control Program running in one host, e.g., host 100, becomes no longer capable of managing new incoming calls because of some buffer storage or Central Processing Unit (CPU) overload. Such an event results in that a user requesting the processing of one supplementary transaction with the considered host 100 will be prevented from communicating with the latter even if another existing host computer could have the needed resources for processing the transactions. In this case, the overload of one host computer affects the overall availability of the network such as appearing to the end user.
The prior art does not provide a communication controller allowing the connection of virtual circuits (VC) according to the X25 recommendations to SNA sessions which has an increased availability and improved re-routing possibilities.